DESCRIPTION

Overview

Marpa parses any language whose grammar can be written in BNF. That includes recursive grammars, ambiguous grammars, infinitely ambiguous grammars and grammars with useless or empty productions.

This document contains a top-level overview of the API for the Marpa parse engine. The two examples in this document show the typical flows of Marpa method calls. This document will use these examples to describe the basic features of Marpa in semi-tutorial fashion. Marpa's advanced features, and full reference details of all features, can be found in the other Marpa API documents.

The three phases

A parser needs to:

Accept a grammar.

Read input.

Return values from the parses, according to a semantics.

In Marpa these three tasks are, for the most part, distinct phases. Grammars are Marpa::XS::Grammar objects. The reading of input and the evaluation of the parse according to the semantics is performed by Marpa::XS::Recognizer objects.

EXAMPLE 1: A SIMPLE CALCULATOR

The synopsis shows the code for a very simple calculator. It handles only addition and multiplication of integers. This section explains, line by line, how it works.

Marpa grammars are Marpa::XS::Grammar objects. They are created with the Marpa::XS::Grammar::new constructor. The arguments to Marpa::XS::Grammar::new are references to hashes of named arguments. In the key/value pairs of these hashes, the hash key is the name of the argument, and the hash value is the value of the named argument.

The start named argument

start => 'Expression',

The start named argument is required. Its value is a string containing the name of the grammar's start symbol.

Named arguments for the semantics

actions => 'My_Actions',
default_action => 'first_arg',

The actions and default_action named arguments specify semantics. Their argument values are strings, which acquire their semantics during evaluation.

Evaluation will be described later. Peeking ahead, the default_action named argument will be interpreted as an action name. This action name will resolve to an action -- a Perl closure that implements semantics. The action specified by default_action is used as the action for rules with no action of their own. actions provides the name of a Perl package where Marpa will look for its actions.

The value of the rules named argument is a reference to an array of rule descriptors. In this example, all the rule descriptors are in the "long" form -- they are references to hashes of rule properties. In each key/value pair of a rule descriptor hash, the key is the name of a rule property, and the hash value is the value of that rule property.

The lhs property

The value of the lhs rule property must be a string containing the name of the rule's left hand side symbol. Every Marpa rule must have a left hand side symbol.

The rhs property

The value of the rhs property is a reference to an array of strings containing names of the rule's right hand symbols, in order. This array may be zero length, in which case this is an empty rule -- a rule with no symbols on the right hand side. There are no empty rules in this example.

The action property

The value of the action rule property is a string. Peeking ahead, each action property string will be interpreted as an action name. This action name will be resolved to a Perl closure that implements the rule's semantics.

Marpa::XS::Grammar::precompute

$grammar->precompute();

Before a Marpa grammar object can be used by a Marpa recognizer, it must be precomputed. Precomputation compiles data structures that the recognizer will need.

Marpa::XS::Recognizer::new

my $recce = Marpa::XS::Recognizer->new( { grammar => $grammar } );

Marpa::XS::Recognizer::new creates a new recognizer. Its arguments are references to hashes of named arguments. In this example the only named argument is the required argument: "grammar". The value of the grammar named argument must be a precomputed Marpa grammar.

Marpa::XS::Recognizer::read

The Marpa::XS::Recognizer::read method takes two arguments, a token name and a token value. The token name must be the name of a valid terminal symbol in the grammar. By default all symbols are valid as terminal symbols, unless a grammar contains empty rules.

The grammars in the examples in this document do not contain empty rules, and therefore are free to use any symbol in the grammar as a token name. For more on terminals, including how to explicitly mark terminal symbols, see "Terminals" in Marpa::XS::Grammar.

The token value must be a Perl scalar, but otherwise its form and semantics are entirely up to the application. If the token value is omitted, it is a Perl undef. In the calculator example, the values of the "Add" and "Multiply" tokens are never used, so they are allowed to default to undef.

Marpa::XS::Recognizer::value

The Marpa::XS::Recognizer::value method returns a reference to the parse result's value, if there was a parse result. If there was no parse result, Marpa::XS::Recognizer::value returns undef.

Resolving the semantics

The first thing Marpa::XS::Recognizer::value needs to do is to resolve the semantics. Resolving the semantics means mapping the action names into actions. Actions are Perl closures which directly implement semantics. In this example, the actions named argument is specified. actions is a Perl package name. Marpa will look for actions in that package.

For example, the action property for the above rule is "do_multiply" and the actions named argument to the grammar was "My_Actions". So Marpa looks for a closure whose fully qualified name is My_Actions::do_multiply, which it finds:

Where there is no action rule property, Marpa tries to use the lhs property as an action name. When Marpa cannot resolve the lhs property as an action name, it will fall back to using the default action for the grammar.

For example, in the first rule in the above display, Marpa will look for a Perl closure with the fully qualified name "My_Actions::Expression". When Marpa does not find a closure by that name, Marpa will fall back to trying to use the default action.

The other two rules in the above display work similarly. Marpa will look for Perl closures named "My_Actions::Term" and "My_Actions::Factor". It will not find them and will fall back to trying to use the default action, as described next.

default_action => 'first_arg',

The default_action named argument is resolved in the same way as are the action properties of the rules. In this example, default_action is specified as "first_arg" and resolves to My_Actions::first_arg.

Actions

Value actions are Perl closures used as callbacks. Value actions are called when nodes in a parse tree are evaluated. A value action receives one or more arguments. The first argument to a value action is always a per-parse-tree object, which the callbacks can use as a scratchpad. In these examples, the per-parse-tree object is not used.

For a non-empty rule, the second and any subsequent arguments to the callback are the values, in lexical order, of the symbols on the right hand side of the rule. If the action is for an empty rule, the per-parse-tree object will be its only argument.

Every value action is expected to return a value. With one exception, this value is passed up to a parent node as an argument. The exception is the value for the start rule. The return value for the start rule becomes the parse result.

Rules with no action specified for them take their semantics from the default_action named argument. If there is no default action for a grammar, rules with no action specified for them return a Perl undef.

EXAMPLE 2: AN AMBIGUOUS PARSE

This is the same calculator as before, rewritten to be ambiguous. Rather than give multiplication precedence over addition, the rewritten calculator allows any order of operations. In this example, the actions (My_Actions::do_add, etc.) and the @tokens array remain the same as before.

Eliminating precedence makes the grammar shorter, but it also means there can be multiple parse trees, and that the different parse trees can have different parse results. In this application we decide, for each input, to return every one of the parse results.

Short form rule descriptors

The rule descriptors in the ambiguous example demonstrate the "short" or array form of rule descriptors. Array form rule descriptors are references to arrays. Here the elements are, in order, the lhs property, the rhs property, and the action property.

Marpa::XS::Recognizer::value

When called more than once, the Marpa::XS::Recognizer::value method iterates through the parse results. For each call, it returns a reference to the parse result. At the end of the iteration, after all parse results have been returned, Marpa::XS::Recognizer::value returns undef. If there were no parse results, Marpa::XS::Recognizer::value returns undef the first time that it is called.

ERRORS AND EXCEPTIONS

Methods in the Marpa API do not return errors. When there are errors, Marpa API methods throw an exception.

INHERITANCE

Classes in the Marpa API are not designed to be inherited.

OTHER DOCUMENTS

This document gives a semi-tutorial overview of the entire Marpa API. For full details on Marpa's grammar objects and their methods, see the Marpa::XS::Grammar document. For full details on Marpa's recognizer objects and their methods, see the Marpa::XS::Recognizer document.

AUTHOR

Jeffrey Kegler

Why is it called "Marpa"?

Marpa is the name of the greatest of the Tibetan "translators". In his time (the 11th century AD) Indian Buddhism was at its height. Marpa's generation of scholars was devoted to producing Tibetan versions of Buddhism's Sanskrit scriptures. Marpa became the greatest of them, and today is known as Marpa Lotsawa: "Marpa the Translator".

ACKNOWLEDGMENTS

Marpa is directly derived from two other parsers. The first was discovered by John Aycock and R. Nigel Horspool and is described in their Aycock and Horspool 2002. The second was described by Joop Leo and is described in Leo 1991. Aycock, Horspool, and Leo, in turn, based their algorithms on the algorithm discovered by Jay Earley. I combined the Aycock-Horspool algorithm with the Leo algorithm, and added significant changes of my own.

I'm grateful to Randal Schwartz for his support over the years that I've been working on Marpa. My chats with Larry Wall have been few and brief, but his openness to new ideas has been a major encouragement and his insight into the relationship between "natural language" and computer language has been a major influence. More recently, Allison Randal and Patrick Michaud have been generous with their very valuable time. They might have preferred that I volunteered as a Parrot cage-cleaner, but if so, they were too polite to say.

Many at perlmonks.org answered questions for me. I used answers from chromatic, Corion, dragonchild, jdporter, samtregar and Juerd, among others, in writing this module. I'm just as grateful to those whose answers I didn't use. My inquiries were made while I was thinking out the code and it wasn't always 100% clear what I was after. If the butt is moved after the round, it shouldn't count against the archer.

In writing the Pure Perl version of Marpa, I benefited from studying the work of Francois Desarmenien (Parse::Yapp), Damian Conway (Parse::RecDescent) and Graham Barr (Scalar::Util). Adam Kennedy patiently instructed me in module writing, both on the finer points and on issues about which I really should have know better.

SUPPORT

Marpa::XS comes without warranty. Support is provided on a volunteer basis through the standard mechanisms for CPAN modules. The Support document has details.

COPYRIGHT AND LICENSE

Copyright 2011 Jeffrey Kegler
This file is part of Marpa::XS. Marpa::XS is free software: you can
redistribute it and/or modify it under the terms of the GNU Lesser
General Public License as published by the Free Software Foundation,
either version 3 of the License, or (at your option) any later version.
Marpa::XS is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser
General Public License along with Marpa::XS. If not, see
http://www.gnu.org/licenses/.